1. Field of the Invention
The present invention relates to a method for producing a nickel metal-hydride storage battery employing a hydrogen-absorbing alloy.
2. Related Background Art
Recently, alkaline storage batteries have been drawing attention as power sources for portable equipment and as power sources for electric cars or hybrid electric vehicles, and are required to have higher performance than ever. Nickel metal-hydride storage batteries, in particular, are secondary batteries that have positive electrodes including an active material containing nickel hydroxide as a principal component and negative electrodes including a hydrogen-absorbing alloy as a principal material. Therefore, they have rapidly spread as secondary batteries having high energy densities and high reliability.
In such a nickel metal-hydride secondary battery, the hydrogen-absorbing alloy has low activity immediately after the battery is assembled. Therefore, there is a problem that the battery power at the initial stage is low. To solve this problem, techniques of activating the hydrogen-absorbing alloy after assembling a battery have been proposed.
For instance, a method of leaving (aging) a battery after assembling for half a day to five days in a state in which a hydrogen-absorbing alloy has absorbed hydrogen has been disclosed (see JP 1(1989)-267966A)
However, the foregoing conventional method has involved a problem that it is difficult to improve the productivity, since the aging requires a long period of time.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a method for producing a nickel metal-hydride storage battery that allows nickel metal-hydride storage battery to be produced with high productivity at low costs.
To achieve the foregoing object, a method for producing a nickel metal-hydride storage battery according to the present invention is a method for producing a nickel metal-hydride storage battery that includes positive electrodes containing nickel hydroxide and negative electrodes containing a hydrogen-absorbing alloy, and has a nominal capacity of C (ampere-hour). The method includes: (i) assembling a battery by enclosing the positive electrodes, the negative electrodes, separators, and an electrolyte in a case; (ii) charging the battery with electric current in a range of 0.05 C (ampere) to 0.2 C (ampere) until a state of charge (hereinafter also referred to as xe2x80x9cSOCxe2x80x9d) rises to a range of 10% to 30%; (iii) overcharging the battery that has been subjected to the charging, with electric current in a range of 0.2 C (ampere) to 1 C (ampere), and thereafter discharging the same until the state of charge lowers to 10% or below; and (iv) subjecting the battery after the discharging to a plurality of charging-discharging cycles. Each charging-discharging cycle is composed of charging the battery with electric current in a range of 0.2 C (ampere) to 5 C (ampere) until the state of charge rises to a range of 60% to 95%, and discharging the same until a battery voltage lowers to a range of 0.70 V to 1.05 V. In this method, in the charging-discharging cycles, the battery is cooled with a coolant at a temperature in a range of 30xc2x0 C. to 60xc2x0 C. It should be noted that the xe2x80x9cstate of chargexe2x80x9d is an index indicative of a charged state, and means an amount of a charge relative to a nominal capacity. The foregoing producing method allows batteries to be activated without aging, thereby making it possible to produce nickel metal-hydride storage battery at high productivity and low costs. The nominal capacity is determined in the following manner. First of all, a battery is charged to 120% of a presumed capacity thereof in an atmosphere at 25xc2x0 C., and thereafter, it is left to stand for 30 minutes. Then, the battery is discharged in an atmosphere at 25xc2x0 C. until the battery has a battery voltage of 1.0 V. The charging and discharging process is carried out with an amperage of one third of the presumed capacity (C/3). This charging and discharging process is repeated twice, and the discharge capacity at the second time is referred to as the nominal capacity.
In the foregoing method, the charging-discharging cycle preferably is carried out 5 times to 60 times. This configuration ensures production of a nickel metal-hydride storage battery that is activated sufficiently.
In the foregoing method, in the charging-discharging cycles, the battery preferably is cooled with a coolant at a temperature in a range of 30xc2x0 C. to 45xc2x0 C. This configuration allows a case made of a resin to be employed.
In the foregoing method, the coolant preferably is water. Water is easy to control in the foregoing temperature range of the coolant, and it is inexpensive as a coolant.
In the foregoing method, in the charging-discharging cycles, the battery preferably is cooled using a cooling frame. This configuration facilitates the cooling of the battery.